PEER-REVIEWED ARTICLE bioresources.com Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2310 Effect of Growth Rings per Inch and Density on Compression Parallel to Grain in Southern Pine Lumber Nathan E. Irby, Frederico J. N. França,* H. Michael Barnes, R. Dan Seale, and Rubin Shmulsky The objective of this study was to evaluate the relationship of growth characteristics to compression properties from commercially available southern pine lumber. The in-grade No. 2 southern pine lumber was collected from retail stores across the Southeast United States. For this specific project, 797 samples of 2 × 8 and 2 × 10 boards were examined. The samples were subjected to static bending following ASTM D 198 to determine mechanical properties. The 2 × 8 samples averaged 4.78 growth rings per inch (RPI) and the 2 × 10s averaged 3.95. Average density (ρ) was 477 kg·m -3 in the 2 × 8 and 487 kg·m -3 for the 2 × 10 specimens. From small clear samples from the parent boards, compression parallel to grain averaged from 43.78 MPa for the 2 × 8 to 46.77 MPa for the 2 × 10. Correlations were run to test significance among growth rings per inch and compressions parallel to grain, across both sizes: 2 × 8 and 2 × 10. Slight significance was found in those correlations and increased in measure from 2 × 8 to 2 × 10. The segmentation of RPI into three distinct groups helped strengthen the findings of effect on compression parallel to grain. Moreover, the addition of density as another test variable further strengthened in relationship per those RPI categorizations. Statistically significant findings for density per RPI segments, R 2 values: for 2 × 8 equal 0.31 (3 or less RPI) to 0.60 (more than 5 RPI). For the 2 × 10 lumber per the same segments, R 2 values: 0.39, 0.46 and 0.25, respectively. The results suggest ρ is a better predictor than RPI alone for compression parallel to grain values. Keywords: Wood mechanics; Lumber; Density; Growth rings Contact information: Department of Sustainable Bioproducts, Mississippi State University, P.O. Box 9820, Starkville, MS 39759 USA; *Corresponding author: [email protected]INTRODUCTION Southern pine comprises a large portion of the Southern United States forest. From the Atlantic coast back along the Gulf of Mexico to Texas, southern pine species loblolly (Pinus taeda L.), shortleaf (P. echinata Mill.), slash (P. elliottii Engelm.), and longleaf (P. palustris Mill.), grow prolifically in natural and artificially generated stands. Historically, mixed oak/gum/pine natural regeneration dominated the re-establishment of recently harvested stands; however, more and more artificially replanted softwood “plantations” have garnered more of the recent regeneration norm (Pawson et al. 2013). Wood anatomical features have been altered in this new forest type, as it affects tree growth (Taylor and Franklin 2011). Visual characteristics of pine strength are a fundamental means for material evaluation in usage classification (Madsen 1976). Number of rings per inch, knots, bark
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PEER-REVIEWED ARTICLE bioresources.com
Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2310
Effect of Growth Rings per Inch and Density on Compression Parallel to Grain in Southern Pine Lumber
Nathan E. Irby, Frederico J. N. França,* H. Michael Barnes, R. Dan Seale, and
Rubin Shmulsky
The objective of this study was to evaluate the relationship of growth characteristics to compression properties from commercially available southern pine lumber. The in-grade No. 2 southern pine lumber was collected from retail stores across the Southeast United States. For this specific project, 797 samples of 2 × 8 and 2 × 10 boards were examined. The samples were subjected to static bending following ASTM D 198 to determine mechanical properties. The 2 × 8 samples averaged 4.78 growth rings per inch (RPI) and the 2 × 10s averaged 3.95. Average density (ρ) was 477 kg·m-3 in the 2 × 8 and 487 kg·m-3 for the 2 × 10 specimens. From small clear samples from the parent boards, compression parallel to grain averaged from 43.78 MPa for the 2 × 8 to 46.77 MPa for the 2 × 10. Correlations were run to test significance among growth rings per inch and compressions parallel to grain, across both sizes: 2 × 8 and 2 × 10. Slight significance was found in those correlations and increased in measure from 2 × 8 to 2 × 10. The segmentation of RPI into three distinct groups helped strengthen the findings of effect on compression parallel to grain. Moreover, the addition of density as another test variable further strengthened in relationship per those RPI categorizations. Statistically significant findings for density per RPI segments, R2 values: for 2 × 8 equal 0.31 (3 or less RPI) to 0.60 (more than 5 RPI). For the 2 × 10 lumber per the same segments, R2 values: 0.39, 0.46 and 0.25, respectively. The results suggest ρ is a better predictor than RPI alone for compression parallel to grain values.
Table 2 shows the results for RPI, ρ, and C∥ for 2 × 8 and 2 × 10 specimens.
Figure 2 shows the relationship between C∥ and RPI of small clear samples obtained from
2 × 8 and 2 × 10 No. 2 southern pine lumber. For 2 × 8 and 2 × 10 lumber sizes exhibited
similar densities (477 and 487 kg∙m-3 respectively). These values are higher than those
found by Kretschmann and Brendtsen (1992) studying 2 × 4 from fast-grown southern
pine (ranging between 390 and 490 kg∙m-3). The 2 × 8 lumber size exhibited slightly
higher variation in RPI compared to 2 × 10. The average value for compression parallel
was higher in 2 × 10 when compared to 2 × 8 lumber size.
Table 2. Overall Results for RPI, ρ, and C∥ of No. 2 2 × 8 and 2 × 10 Southern Pine Lumber Samples
Lumber Size (in x in)
n
RPI ρ*
(kg∙m-3) C∥
(MPa)
2 × 8 472
Average 4.78 477 43.78
Min 1.70 343 24.54
Max 19.00 701 68.99
COV 57.47 12.72 18.98
2 × 10 302
Average 3.95 487 46.77
Min 1.67 352 25.59
Max 16.33 657 81.24
COV 55.09 13.34 20.72 *ρ = density (kg∙m-3).
Although the data appeared to be widely varying, the conglomeration of data
points was significant for 2 × 8 with an R2 of 0.10 (Fig. 2a) and for 2 × 10 with an R2 of
0.14) both at p = 0.05 level (Fig. 2b).
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2314
(a)
(b)
Fig. 2. Correlation between compression parallel to grain and growth rings per inch for: (a) 2 × 8; and (b) 2 × 10 No. 2 southern pine lumber
As the previous correlations indicated, only slightly casual relationships in RPI to
compression parallel to grain values, further refinement of the data was needed to discern
the relationship. A data segmentation approach categorized the growth rings per inch data
into three distinct groups for further analysis. Those groups consisted of: 1) <3 RPI; 2)
3≤5 RPI; and 3) >5 RPI. Table 3 shows the data breakout per RPI groups and the test
variables for the 2 × 8 and 2 × 10 samples, plus statistical output.
y = 1.04x + 39.19R² = 0.10
0
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0 5 10 15 20
Co
mp
ressio
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(MP
a)
Growth Rings per Inch (RPI)
2 × 8
y = 1.66x + 40.14R² = 0.14
0
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0 5 10 15 20
Co
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a)
Growth Rings per Inch (RPI)
2 × 10
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2315
Table 3. Growth Rings per Inch Categories, Compression Parallel to Grain Averages per RPI group, Coefficients of Determination for 2 × 8 and 2 × 10 Lumber, and p-values.
Growth Ring Category
n C∥ (MPa) Coefficient
of Correlation
R2 p-value
2 x 8
< 3 RPI 160 40.82 0.281 0.079 <0.001*
3 ≤ 5 RPI 214 43.16 0.241 0.058 <0.001*
> 5 RPI 98 49.51 0.045 0.002 0.650ns
2 x 10
< 3 RPI 141 43.28 0.138 0.019 0.103ns
3 ≤ 5 RPI 102 47.82 0.032 0.001 0.796ns
> 5 RPI 59 52.95 0.176 0.031 0.181ns *significant at α=0.05; ns not significant
The correlations for 2 × 8 lumber showed positive correlations for the less than 3
RPI group and the three to less than or equal to five RPI group. None of the groups for 2
× 10 lumber showed a significant relationship between RPI and compression parallel to
grain. As correlations are often sensitive to outliers, the data could be scrubbed further.
For instance, in the 2 × 10 dataset, the highest RPI was 16, with a few in the 15 RPI
range, pulling the data.
Density was then used as an independent variable to see if a stronger relationship
was present. The initial test for compression parallel to grain as the dependent variable
and using density as the independent was done across all data for each lumber size, 2 × 8
(Fig. 3a) and 2 × 10 (Fig. 3b). When density was used as an independent variable, R2
values increased to 0.32 for 2 × 8 lumber and 0.35 for 2 × 10 lumber.
(a)
y = 0.08x + 6.56R² = 0.32
0
10
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30
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50
60
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80
300 350 400 450 500 550 600 650Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg ·m-3)
2 × 8
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2316
(b)
Fig. 3. Correlation between compression parallel to grain and density for: (a) 2 × 8; and (b) 2 × 10 No. 2 southern pine lumber
Similar to Table 3 for the growth rings per inch comparison with compression
parallel to grain, density was also analyzed in the same fashion. Table 4 illustrates the
relationship between density and C∥ for 2 × 8 and 2 × 10 lumber sizes.
Table 4. Density and C∥ Correlation Coefficients and R2 values for 2 × 8 and 2 × 10 Lumber
Independent Variable
n C∥ (MPa) Coefficient
of Correlation
R2 p-value
2 x 8
Density (kg∙m-3) 472 43.78 0.57 0.32 <0.001*
2 x 10
Density (kg∙m-3) 302 46.77 0.59 0.35 <0.001* *significant at α=0.05;
Further tests were done to understand better the density variation in 2 × 8 and 2 ×
10 lumber sizes. Table 5 shows the attempt to discern any mean differences between the
two lumber sizes. The t test reveals significant difference between the two means at the
0.05 level (p-value < 0.001).
Table 5. Two Sample t Test for Compression Parallel to Grain of Small Clear Samples Taken from Parent 2 × 8 and 2 × 10 No. 2 Southern Pine Lumber
Lumber Size
n C∥ mean (MPa) COV t test p-value
2 x 8 472 43.74 14.70 4.52 < 0.001*
2 x 10 302 46.70 20.77 *significant at α=0.05;
y = 0.09x + 4.03R² = 0.35
0
20
40
60
80
100
300 350 400 450 500 550 600 650
Co
mp
ress
ion
par
alle
l (M
Pa)
Density (kg ·m-3)
2 × 10
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2317
The next analysis evaluated density as an independent variable but used the
segmentations of the data based on RPI groups: less than 3 RPI, 3 ≤ 5 RPI, and more than
5 RPI. Figure 4 a, b, and c show those values for 2 × 8 lumber across ρ/RPI segments, R2
= 0.31, 0.19, 0.60, respectively.
(a)
(b)
(c)
Fig. 4. Correlation between compression parallel to grain and density for 2 × 8 southern pine lumber derived from growth rings per inch segmentation of: (a) 3 RPI; (b) 3 ≤ 5 RPI; and (c) more than 5 RPI
y = 0.08x + 4.64R² = 0.310
20
40
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80
300 400 500 600
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg ·m-3) per RPI segment ≤ 3
2 × 8
y = 0.06x + 16.36R² = 0.190
10
20
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60
70
350 450 550 650Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg·m-3) per RPI segment 3 ≤ 5
2 × 8
y = 0.10x - 2.28R² = 0.600
20
40
60
80
350 450 550 650
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg·m-3) per RPI segment > 5
2 × 8
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Subsequent analysis of density and the RPI segments for the 2 × 10 data was also
evaluated. Figure 5 a, b, and c show those values for 2 × 10 lumber across ρ/RPI
segments, R2 = 0.29, 0.39, 0.20, respectively.
(a)
(b)
(c)
Fig. 5. Correlation between compression parallel to grain and density for 2 × 10 southern pine lumber derived from growth rings per inch segmentation of: (a) 3 RPI; (b) 3 ≤ 5 RPI; and (c) more than 5 RPI
y = 0.08x + 7.42R² = 0.29
0
20
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60
80
100
300 400 500 600 700
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) per RPI segment ≤ 3
2 × 10
y = 0.09x + 1.70R² = 0.39
0
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50
60
70
80
300 400 500 600 700
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) per RPI segment 3 ≤ 5
2 × 10
y = 0.04x + 32.37R² = 0.20
0
10
20
30
40
50
60
70
80
350 450 550 650
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) per RPI segment > 5
2 × 10
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Lumber pieces with more than 5 RPI exhibited higher R2 values in both lumber
sizes. This occurs due to the higher amount of mature wood in the pieces and a more
homogenous wood material. Kretschmann and Bendtsen (1992) found lower number of
RPI (between 3.7 and 4.4) compared to this study. The material used by the authors had a
greater amount of juvenile wood.
The third analysis evaluated segmentations of the density data in increments of <
0.4, 0.4 ≤ 0.5, and ≥0.5. Figure 6 a, b, and c show those relationships for 2 × 8 lumber. R2
= 0.19, 0.22, and 0.03 respectively.
(a)
(b)
y = 0.21x - 44.50R² = 0.19
0
10
20
30
40
50
60
70
340 350 360 370 380 390 400
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg · m-3) ≤ 400
2 × 8
y = 0.11x - 7.60R² = 0.22
0
10
20
30
40
50
60
70
390 410 430 450 470 490 510
Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg·m-3) 400 ≤ 500
2 × 8
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2320
(c)
Fig. 6. Correlation between compression parallel to grain and density segmentation of: (a) less than 0.4; (b) 0.4 ≤ 0.5, and (c) more than 0.5 (ρ) for 2 × 8 southern pine lumber
Figures 7 a, b, and c show those relationships for 2 × 10 lumber from smallest
density segment less than 0.4 to more than 0.5 density group: R2 = 0.25, 0.08, and 0.46
respectively.
(a)
y = 0.04x + 29.04R² = 0.03
0
10
20
30
40
50
60
70
80
490 540 590 640 690Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg·m-3) > 500
2 × 8
y = 0.19x - 35.01R² = 0.25
0
10
20
30
40
50
340 350 360 370 380 390 400Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) ≤ 400
2 × 10
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Irby et al. (2020). “Growth rings & pine properties,” BioResources 15(2), 2310-2325. 2321
(b)
(c)
Fig. 7. Correlation between compression parallel to grain and density segmentation of: (a) less than 0.4; (b) 0.4 ≤ 0.5, and (c) more than 0.5 (ρ) for 2 × 10 southern pine lumber
To summarize the findings, Table 6 shows R2, standard error and p-value for each
test across all data and segmentations. The full list of independent variables is listed in
the first column. Lumber sizes are represented by rows. Slope, intercept, R2, standard
error of estimate and p-value are for compression parallel test and the various
independent variables. For 2 × 10 lumber exhibited a stronger relationship between
compression parallel and the independent variables. Density as an independent variable
predicts CII better than RPI.
y = 0.07x + 12.37R² = 0.08
0
10
20
30
40
50
60
70
380 400 420 440 460 480 500 520Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) 400 ≤ 500
2 × 10
y = 0.09x + 6.97R² = 0.46
0
10
20
30
40
50
60
70
80
350 400 450 500 550 600 650 700Co
mp
ressio
n p
ara
llel
(MP
a)
Density (kg· m-3) per RPI segment > 5
2 × 10
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Table 6. Summary of R2 Values for Tests of C∥ and the Various Independent Variables
Independent Variable
Slope (m)
Intercept (b)
Coefficient of Determination (R2)
Standard Error of Estimate
p-value
2 × 8
RPI 1.04 39.19 0.10 0.14 <0.001
ρ * 0.08 6.56 0.32 0.01 <0.001
ρ/RPI < 3 RPI
0.08 4.64 0.31 0.01 <0.001
ρ/RPI 3 ≤ 5 RPI
0.06 16.36 0.19 0.01 <0.001
ρ/RPI > 5 RPI
0.10 -2.28 0.60 0.01 <0.001
ρ < 0.4 0.21 -44.50 0.19 0.07 0.01
ρ 0.4 ≤ 0.5 0.11 -7.60 0.22 0.01 <0.001
ρ > 0.5 0.04 29.04 0.03 0.02 0.03
2 ×10
RPI 1.66 40.14 0.14 1.65 <0.001
ρ * 0.09 4.03 0.35 0.09 <0.001
ρ/RPI < 3 RPI
0.08 7.42 0.29 0.01 <0.001
ρ/RPI 3 ≤ 5 RPI
0.09 1.70 0.39 0.01 <0.001
ρ/RPI > 5 RPI
0.09 6.97 0.46 0.01 <0.001
ρ < 0.4 0.19 -35.01 0.25 0.07 0.02
ρ 0.4 ≤ 0.5 0.07 12.37 0.08 0.02 <0.001
ρ > 0.5 0.10 -2.71 0.16 0.02 <0.001 *ρ = density (kg∙m-3). The coefficients m and b are used in the generalized model C∥ = m
(independent variable) + b.
Further refinement of the data, removal of blatant outliers, and combinations of
other growth characteristics could be explored. Subsequent analyses could be conducted
using a less stringent alpha significance level and perhaps show more association among
the variables at lower confidence intervals.
CONCLUSIONS
1. The 2 × 8 and 2 × 10 specimens utilized in this study exhibited average
growth characteristics conducive to the lumber standard for No. 2 grade
southern pine (SPIB 2014).
2. The correlation of growth rings per inch in relation to compression parallel
to grain were only slightly significant, and also increased in value from
smaller to larger size sample types for the scatter plots.
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3. Once the RPI data were segmented, statistical significance showed in the
first two groups for 2 × 8 lumber but for none of the groups for 2 × 10
lumber.
4. Because of widespread data distribution and too many outliers could be
attributed, in addition to small sample size for at least one of the segments.
Further refinement of the data, removal of blatant outliers, and
combinations of other growth characteristics could be explored. With that
premise, density was added as an independent variable and segmented in
two different ways: the first segmentation was done using the RPI group
data, and the second utilized density segments from less than 0.4, 0.4 ≤
0.5, and more than 0.5.
5. The second class of segmentations test, the density groups did not fare as
strongly, especially for 2 × 8 lumber.
ACKNOWLEDGMENTS
The author wishes to acknowledge the U.S. Department of Agriculture (USDA);
Research, Education and Economics (REE); Agriculture Research Service (ARS);
Administrative and Financial Management (AFM); Financial Management and
Accounting Division (FMAD); and Grants and Agreements Management Branch
(GAMB) under agreement no. 58-0202-4-001. In addition, special thanks as well to the
USDA Forest Service Forest Products Laboratory; Southern Pine Inspection Bureau and
Timber Products Inspection for the assistance in the larger study from which this project
stemmed. This paper was approved as journal article SB 955 of the Forest & Wildlife
Research Center, Mississippi State University.
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